Several steps in the manufacture of semiconductor devices require the removal of photoresist (PR) and other materials from substrates. The "removal" of PR is synonymous with "ash," "burn," "strip," and "clean." One example of a substance that may be removed is a residue that may remain on a silicon wafer after an etching step is completed. Such residue is frequently composed of a polymeric material that may be present in the form of "veils" and is caused by over etching during etch processes.
One way to remove PR and other materials such as those described above is by directing a plasma stream at the substrate with the attached substance. In some cases, the substance is removed from the substrate by the "afterglow" of the plasma rather than the plasma itself.
One gas that may be used in generating the plasma to strip the substrates is oxygen. In order to increase the effectiveness and/or efficiency of processes for removing materials from a substrate, a source of fluorine may be added to the oxygen.
A problem inherent in utilizing a fluorine-containing substance is degradation of the plasma generating/discharge device by the fluorine. For example, plasma generating and discharge devices typically include a tube through which the plasma flows. Microwave or radio-frequency energy is introduced into the tube to excite the gas and form the plasma. Typically, the tube is made of quartz. It was observed that fluorine was particularly destructive to the quartz tubes. Therefore, quartz tubes were replaced by sapphire tubes in order to overcome the corrosive effects of fluorine.
Although the sapphire plasma discharge tube included in the plasma generating and discharge devices overcame the significant degradation of the quartz tubes, ash rates of photoresist utilizing the sapphire tube were markedly lower than ash rates observed when utilizing the quartz tubes.
One reason for the decreased efficiency of ash by an oxygen plasma generated with a plasma generating and discharge device that includes a sapphire tube is that oxygen atoms in the plasma may be lost at the surface of the sapphire to a much greater degree than at the surface of the quartz.